In recent years the theory that measurement of the potential level of the electromagnetic field of a living organism can be used as an accurate diagnostic tool is gaining greater acceptance. Many methods and devices for diagnosing diseases have been developed in an attempt to implement this theory. For example, U.S. Pat. No. 4,328,809 to B. H. Hirschowitz et al. deals with a device and method for detecting the potential level of the electromagnetic field present between a reference point and a test point of a living organism. Here, a reference electrode provides a first signal indicative of the potential level of the electromagnetic field at the reference point, while a test electrode provides a second signal indicative of the potential level of the electromagnetic field at the test point. These signals are provided to an analog-to-digital converter which generates a digital signal as a function of the potential difference between the two, and a processor provides an output signal indicative of a parameter or parameters of the living organism as a function of this digital signal.
Similar biopotential measuring devices are shown by U.S. Pat. Nos. 4,407,300 to Davis, and 4,557,271 and 4,557,273 to Stoller et al. Davis, in particular, discloses the diagnosis of cancer by measuring the electromotive forces generated between two electrodes applied to a subject.
Unfortunately, previous methods for employing biopotentials measured at the surface of a living organism as a diagnostic tool, while basically valid, are predicated upon an overly simplistic hypothesis which does not provide an effective diagnosis for many disease states as well as other trauma causing conditions. Prior methods and the devices which implement them operate on the basis that a disease state is indicated by a negative polarity which occurs relative to a reference voltage obtained from another site on the body of a patient, while normal or non-malignant states, in the case of cancer, are indicated by a positive polarity. Based upon this hypothesis, it follows that the detection and diagnosis of disease states can be accomplished by using one measuring electrode situated on or near the disease site to provide a measurement of the polarity of the signal received from the site relative to that from the reference site. When multiple measuring electrodes have been used, their outputs have merely been summed and averaged to obtain one average signal from which a polarity determination is made. This approach is subject to major deficiencies which lead to diagnostic inaccuracy.
First, the polarity of diseased tissue underlying a recording electrode has been found to change over time. This fact results in a potential change which confounds reliable diagnosis when only one recording electrode is used. Additionally, the polarity of tissue as measured by skin surface recording is dependent not only upon the placement of the recording electrode, but also upon the placement of the reference electrode. Therefore, a measured negative polarity is not necessarily indicative of diseases such as cancer, since polarity at the disease site depends in part on the placement of the reference electrode.
As disease states such as cancer progress, they produce local effects which include changes in vascularization, water content, and cell division rate. These effects alter ionic concentrations which can be measured at the skin surface. Other local effects, such as distortions in biologically closed electrical circuits, may also occur. A key point to recognize is that these effects do not occur uniformly around the disease site. For example, as a tumor grows and differentiates, it may show wide variations in its vascularity, water content and cell division rate, depending on whether examination occurs at the core of the tumor (which may be necrotic) or at the margins of the tumor (which may contain the most metabolically active cells). Once this fact is recognized, it follows that important electrical indications of disease are going to be seen in the relative voltages recorded from a number of sites at and near a diseased area, and not, as previously assumed, on the direction (positive vs. negative) of polarity. Methods and devices for effectively performing such disease diagnosis and screening have been developed as illustrated by U.S. Pat. Nos. 4,955,383 and 5,099,844 to M. L. Faupel.
For all biopotential measurements where DC electrical signals are recorded using tissue contacting electrodes, regardless of the measuring instrumentation and method employed, the accuracy of the resulting measurement is extremely dependent upon the electrodes used and the presence or absence of DC offset potentials in these electrodes. Small DC offset potentials can be tolerated in electrodes used to sense AC potentials, such as those employed for electrocardiograms, but where the biopotentials sensed are small DC potentials, DC offset potentials in the electrodes of only a few millivolts can significantly affect the accuracy of any measurement taken with the electrodes.
Many electrodes are packaged in a pre-gelled state wherein an electrolytic paste or gel is packaged as part of the electrode. The gel may be located in a central gel reservoir consisting of a molded cup, or it may be contained in a dye-cut hole in a foam which encapsulates a gel saturated open cell compressible foam column, such as U.S. Pat. No. 3,868,946. In most instances, the pre-gelled electrodes are sold ready for use with an electrically conductive material such as metal or a metal chloride in contact with the electrolyte gel.
A pre-gelled electrode is a battery by itself, but the battery effect cannot be measured unless two such electrodes are placed face to face with the gels for each electrode in contact relationship. In the use of such electrodes, a complex battery is formed consisting of many interactive components including the electrode material (frequently silver/silver chloride), the electrode gel, internal body chemistry and external skin conditions, skin preparation, temperature, air condition and chemistry, etc. Obviously, some of these factors are not subject to control, but in order to get the best data possible, especially in instances where DC biopotentials are of interest, artifacts, such as DC offsets, should be reduced to the lowest level. Clearly, pre-gelled electrodes can possibly present such undesired DC voltage artifacts which should be limited to the lowest voltage possible; ideally zero volts. Most pre-gelled electrodes when introduced in the battery system outlined above contribute some unwanted DC voltage (polarization effect) to biopotential measurement. This is particularly true when two or more pre-gelled electrodes are packaged in face to face contact with the electrolyte gel of opposed electrodes in contact as illustrated by U.S. Pat. No. 4,034,854, to A. J. Bevilasqua, for now a true battery, is formed and polarization will occur. It is important to lower the possibility of such DC artifacts occurring in a degree sufficient to have a substantial adverse effect on DC biopotential measurements, and U.S. Pat. No. 5,217,014 entitled Depolarized Pre-Gelled Electrodes discloses methods and various devices for effectively accomplishing this purpose.
Even when the most accurate and advanced instrumentation for measuring and diagnosing or screening for a disease or tissue injury condition is used, inaccurate readings can result if the electrodes connected to the measuring instrument cannot provide a signal which accurately represents a sensed DC biopotential. Since the electrodes used are replaceable electrodes, it is important that before initiating each DC biopotential measurement operation, it is positively determined that proper electrodes with an acceptable DC voltage artifact are connected to the measuring instrument.